The present invention relates to a tug boat intended for escort towing and/or for use in harbor and including at least a towing winch mounted on the fore-castle. While escort towing, the tug boat is intended to assist at high speed the steering and arresting properties of a vessel to be assisted by means of a tow rope coming from the towing winch and connected to the vessel being assisted. While working in the harbor, the tug boat can be applied to normal towing and buffering tasks.
Accidents have occurred in the immediate past, which may even have lead to major oil damages, which accelerated pressure toward improvements in safety in marine oil transports. Some of the accidents lead to oil damage which resulted from an oil tanker that lost either its steerability or propulsive thrust at a critical moment. As a consequence of such oil accidents, the requirements concerning tanker structures have been tightened, inter alia, so that a double bottom structure is required to be built in tankers. In addition, development of tug boats of a novel type has been necessary to provide assistance to and escort tankers in dangerous and coastal waters, i.e., outside of safe harbors.
Totally different standards are set for such, so-called escort tug boats compared with conventional harbor tug boats. First, the escorting speed of an escort tug boat is required to be at least as high as the lowest operating speed of a tanker. The most economical escorting speed is the highest permitted operating speed for tankers in a certain area, or, if no such limitations exist, the highest permitted speed at which the trafficking is safe. In practice, this means that the escorting speed can be even 13 to 14 knots. Accordingly, the tug boat is required at this speed to be able to carry out its escorting tasks as well as merely following the tanker at this speed. Furthermore, the escort tug boat should be able to function in all weather conditions. Such prerequisites mandate that an escort tug boat should be able to function in all conceivable directions and, if needed, it has to be able to change the direction at maximum speed. Furthermore, an escort tug boat like this is required to possess maximum traction power. In view of such requirements, the only useful propulsion apparatus in current escort tug boats is, in fact, a propeller means capable of turning around 360.degree. and possessing a great propulsive thrust.
Primarily two types of tug boats appropriate for escort towing are known in the art, one of them being a so-called tractor tug boat in which the towing winch is positioned on the aft deck and in which the propeller means have been disposed on the front side to the towing winch, closer to the bow of the vessel. The other type is a so-called stern drive tug boat in which the towing winch is placed on the fore deck and in which the propeller means have been arranged in the stern of the vessel. The tractor tug boats and escort stern drive tug boats thus represent the state of art technology. A drawback particularly related to the stern drive tug boats is that although the lateral surface area of the hull thereof is rather large, it is not advantageous as far as its shape is concerned and the point of application of the force is located too far back so that transverse forces are difficult to achieve.
In ordinary tug boats, which are mainly intended for towing only and not for arresting, an arcuate construction provided with a hook is generally arranged on the aft deck of the tug boat to which hook, the tow rope is fastened. This construction has been found to increase the stability of the tug boat. On the forecastle of tug boats intended for arresting, no such constructions have been used.
On the other hand, a box keel or plate keel has frequently been used to improve the direction stability in ordinary vessels, but not in tug boats.